The goal of our studies is to understand the molecular mechanisms of paramyxovirus membrane fusion using Newcastle disease virus as a prototype. As in most paramyxovirus systems, membrane fusion requires both HN and F proteins. Our studies have focused on two questions, how the F protein mediates membrane fusion and the role of RN protein in that process. Our mutational analysis of the F protein has shown that two heptad repeat domains, HR1 and HR2, are important in the fusion activity of the protein. We have shown that peptides with sequences from these domains interact but likely only upon activation of fusion. We have also found that sequences from the HN protein will interact with a specific domain within the F protein. Furthermore, we have made a point mutation within another F protein heptad repeat domain, HR3, which eliminated the absolute requirement for RN protein in fusion. To address the questions raised by these findings we propose four specific aims: 1) To determine limits of HN protein and F protein interacting domains. 2) To characterize the interactions of peptides and peptide analogues from the HR1, HR2, and RN sequences in order to clarify mechanisms involved in fusion inhibition by F mutants, to identify residues important for conformational shifts, and to identify residues important in RN-F protein interactions. 3) To define the nature of conformational shifts in the glycoproteins using peptides, peptide analogues, as well as peptide specific antisera. 4). To explore the role attachment in initiation of fusion.